1. Field of the Invention
This invention relates to a reversible thermosensitive material which permits repeated formation and erasure of images by utilizing a reversible change in transparency of a thermosensitive layer thereof with temperature.
2. Discussion of Background
A variety of information recording media have been developed to comply with the demands of the expanding and diversifying volume of information. A reversible thermosensitive recording medium has been attracting much attention recently for its capability of forming and erasing images which can be carried out repeatedly on demand. Reversible thermal recording media are known to be capable of forming and erasing images by reversibly changing optical properties between two states, a white opaque state (image formed) and a transparent state (erased), depending on temperature.
As representative examples of this kind of reversible thermosensitive recording materials, there have been disclosed in Japanese Laid-Open Patent Applications (referred to as "JPA", hereinafter) 54-154198 (1979) and 55-154198 (1980), recording materials having a thermosensitive layer whose transparency reversibly changes depending on the temperature and in which a low molecular weight organic material is dispersed in a matrix resin such as polyester, polyvinyl chloride or vinyl chloride-vinyl acetate copolymer.
Such recording materials, however, have the disadvantage that the optical density of recorded images decreases with the repetition of recording and erasing, thereby resulting in a decrease in image contrast by repeated heat applications with a heating element such as, for example, a thermal print head used for carrying out the formation and erasure of the recorded images.
In view of the above problem, JPA-62-154547 (1987) proposes the use of a resin matrix for the recording material, having a selected average degree of polymerization and also a selected repetition unit of the vinyl chloride in the copolymer (in particular, an increased average degree of polymerization), to thereby improve the thermal resistance of the matrix and the durability of the recording materials.
Also, JPA-5-85045(1993) proposes to use a resin matrix for the recording material, which is composed of heat cured hydroxylated vinyl chloride-vinyl acetate copolymer with added isocyanate, to thereby improve the thermal resistance of the matrix and the durability of the recording materials.
Such recording materials, however, have shortcomings in that (1) because of the relatively narrow energy range in which this material can be erased, it is not possible to erase an image completely by heating for a short period of time with a thermal printhead, (2) when a white opaque image is stored for a long time, the erasure characteristics often change so that image erasure under conditions suitable in the early stage of the storage becomes incomplete and (3) the difficulty mentioned just above is found more pronounced with increase in the storage temperature.
In addition, other disadvantages are also realized for known reversible thermosensitive recording media in that (1) portions of the recording layer stick to a heating element such as a thermal printhead, caused by a high friction therebetween and (2) the surface of the recording layer becomes rough, having an approximately periodic feature formed corresponding to the dot density of the thermal printhead, which is caused by the relatively easily deformable nature of the layer and increases with the number of repetitions of heat application, thereby resulting in deterioration of the quality of images formed on the reversible thermosensitive recording medium.
To obviate the above-mentioned problem, several attempts have been made to use an overcoat layer disposed on the recording layer, to thereby reduce the coefficient of friction of the recording medium. Illustrative examples of the overcoat layer are one composed of silicone resin or silicone rubber, disclosed in JPA-62-55650 (1987); another overcoat layer of polysiloxane graft polymer, in JPA-63-221087(1985) ; a heat-resisting lubricant coat of silicone graft polymers as a major ingredient, in JPA-2-86491(1990); an overcoat of polysiloxane-polyurea-polyamide multi-block copolymer, in JPA-8-11439(1996); and an overcoat layer of iminohydantoin copolymer including silicon, in JPA-8-11440(1996).
These overcoat layers, however, have a drawback in that it is difficult, for use in offset printing and gravure printing, to transfer printing ink onto the overcoat layer satisfactorily, and to provide printed images on the layer with satisfactory characteristics such as scratch resistance and anti-peeling property against an adhesive tape.
In addition, the above-mentioned overcoat layer of JPA-62-55650 is found to have the drawback of an unsatisfactory adhesive property between it and the underlying recording layer, resulting in peeling caused by repeated application of mechanical stress during the imaging processes and thereby in reduced quality of recorded images. To improve this adhesive property, JPA-1-133781 (1989) discloses a layered structure for the recording medium, in which an intermediate layer composed primarily of resin is formed on a reversible thermosensitive recording layer, and an overcoat layer of heat-resistant resin is formed on the intermediate layer. By this expedient, the adhesive property is found improved by the intermediate layer and the deformation of the surface of the recording media is reduced by the overcoat layer.
In this case, however, there still remain disadvantages such as scratches on the recording layer and removal of portions of the overcoat layer which then stick to heating elements of the thermal printhead, caused by repeated application of mechanical stress during imaging processes. With the increase in accumulation of these portions on the heating elements, the heat conduction from the heating elements to the recording media is hindered, thereby resulting in difficulty in forming satisfactory images.